The field of this disclosure is making three-dimensional topographic structures by means of graduated exposure in a positive-tone photosensitive material, such as a photoresist, photosensitive polymide, or similar. Typically the produced surfaces have a surface profile which is non-reentrant, i.e. for each lateral point (x, y) the surface has only one height z(x,y) or there may be points where the surface is approximately vertical (perpendicular to the xy plane). Alternatively they may be said to have only positive slopes (including approximately 90 degrees to the xy plane, but no significantly negative (overhanging) slopes. The surfaces may be called 2.5D surfaces since they have more dimensions than the xy plane, but are significantly more constrained than a 3D surface. Many relevant surfaces will have only positive slopes.
Such 2.5D patterns may be written in positive resist either to be used directly as optical, mechanical, fluidic, etc. components, e.g. diffusors, non-reflecting surfaces, Fresnel lenses and Fresnel prisms, computer-generated holograms, lenslet arrays, etc, or to be used as masters for the fabrication of such components by replication. Replication can be done by molding, pressing, embossing, electroplating, etching, as known in the art.
Useful and compact introductions to resist exposure and development can be found at http://www.microchemicals.eu/technical-information, especially the articles entitled “Exposure of Photo Resists,” 5 pp. revised 2007 Mar. 12; “Optical Parameters of Photoresists,” 2 pp., revised 2007 Feb. 26; and “Development of Photoresists,” 3 pp., revised 2007 Feb. 28.
FIG. 1 shows a process for creating a 2.5D surface structure on a workpiece by means of varying exposure of a photoresist as known in the art. In FIG. 1a, a positive-tone photoresist 101 is applied to a workpiece 102. In FIG. 1b, the resist is exposed to electromagnetic radiation 103 with higher 104 and lower 105 exposure dose in an exposure system 106. In FIG. 1c, the developer 107 dissolves part of the resist. Areas exposed to a higher dose 104 dissolve faster than areas exposed with less dose 105, creating a three-dimensional surface pattern 108, as depicted in FIG. 1d. The profile can be used directly (as shown in FIG., scattering light in a controlled fashion.) It can be transferred into a material with more durable or otherwise more suitable properties 109, as in FIG. 1f. It can be used for replication of the three-dimensional pattern 110, as shown in FIG. 1g. 
Positive tone in this disclosure means that the developer removes resist that is exposed above a certain dose, the threshold. Resists with high contrast (high gamma) have a sharp on-set of dissolution at the threshold dose, while for resist with low contrast (low gamma) the dissolution rate is more proportional to the dose. This is illustrated in the article, “Exposure of Photo Resists,” supra, available at http://www.microchemicals.eu/technical-information. The article posits that grey scale lithography uses low contrast resist, rather than high contrast resist.
Negative-tone resists, e.g., SU-8, become insoluble with increasing exposure dose. Since there is always some absorption in the resist the dose is higher at the top surface than close to the substrate, and it is only when the resist is fully exposed that it will adhere to the substrate after development. Partially exposed features or areas will fall off or peel during development or rinsing. Therefore negative-tone resists tend to be less suitable to the writing of 2.5D surfaces.